Mystery ‘Concord' mixer amplifier with mystery output transistors

Hello,
I am trying to fix this mixer/amplifier:

IMG_20230608_200638083.jpg


Unfortunately, I can't find any information on it, let alone a schematic. does anyone have a clue?

The problem seems to be the output transistors, at least one of them (the NPN one) measures short circuit between collector and emitter. Can't be good. But here to, I am unable to find any reference. Here they are:

IMG_20230608_200752270.jpg



Looking at the PCB, I see no driver transistors, so I think the above individuals are darlingtons. But how to know? with my multimeter on diode check, the readings are:

NPN:

C to E short circuit
B to E 0.6v
B to C 0.6v
E to B 1.8v
C to B 1.8v

PNP:

C to E infinite
E to B infinite
C to B 0.6v
B to E infinite
B to C 1.5v
E to C infinite


Any help is very welcome,

Gijser
 
Normally the B/E junction would drop around 1.2 volts forward biased.
Yes, that's what I thougt, so the PNP can also be considered kaputt I guess, since it has no voltage drop whatsoever between B and E.
Thank you for your replacement suggestions, I had actually looked at a pair Mj4031/4034, they are still in production in the far east and inexpensive too. It is reassuring you come up with that same device! So I think I will try those. And I need to replace the emitter resistor of the NPN device, it was fried to open circuit.
 
I don’t think ST makes any of those old TO-3’s anymore. You should be able to get Mospec or Central from regular distribution channels. Sometimes the old 2N versions like 2N6050, 6052 show up on the surplus market. I buy and try, but haven’t gotten burned by fakes on those types from those sources (they have a hard time unloading them, no real market for fakes).

MJ11015, 11016 still available from On, if you can find stock and are willing to pay over ten bucks a pop. Also, way overkill.
 
Above list link was for TO-126.
Also, OP is in Europe, so BD series may have something useful, though in this age that is not very important...

Can drill / tap and fix to heat sink, use insulated wires, if needed, provided there is space.
Then the package is not crucial, heat dissipation and other parameters should be adequate...see the below post.
Finding and fixing the fault is important.
 
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Of course you have to fix the fault. But you also need replacements that are good for 100W at 4 ohms probably on +/-40V rails. TO-220’s have a hard time getting out more than 20W of bulk heat - a recipe for blowing two more transistors. 3P’s can deal with 40, which is enough dissipation. If you can get the metal cans reasonably that would be better - just drop them right in.
 
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Yes, that's what I thougt, so the PNP can also be considered kaputt I guess, since it has no voltage drop whatsoever between B and E.
Worth trying (for curiosity) a 9v battery and say a 1 or 2k series resistor across B and E and see what actual volt drop you see. In any case you're going to replace as a pair but its always interesting to see what has happened.

I'd also use a bulb tester when powering up in case of other issues.
 
I don’t think ST makes any of those old TO-3’s anymore. You should be able to get Mospec or Central from regular distribution channels. Sometimes the old 2N versions like 2N6050, 6052 show up on the surplus market. I buy and try, but haven’t gotten burned by fakes on those types from those sources (they have a hard time unloading them, no real market for fakes).

MJ11015, 11016 still available from On, if you can find stock and are willing to pay over ten bucks a pop. Also, way overkill.
CDIL from India and Iscsemi from China manufacture many obsolete TO3 and other transistors. Here in Europe they are readily available, don't know about USA. The MJ11015 you can buy the CDIL version for about 2 dollars, against 10 dollars for the Onsemi part. I have used CDIL parts before, and they are good quality and consistent.
Worth trying (for curiosity) a 9v battery and say a 1 or 2k series resistor across B and E and see what actual volt drop you see. In any case you're going to replace as a pair but its always interesting to see what has happened.

I'd also use a bulb tester when powering up in case of other issues.
I will try that. Actually, I have fired up the amp without the output transistors in place, and power transformer and voltages seem ok. +- 40 volt rails.

The amp has a switch at the back for 4-8-16 ohms. It has two output jacks, wich are wired parallel, but this is not indicated on the back panel. So maybe someone connected two big 8 ohm speakers, resulting in 4 ohm load, and left the switch on 8 ohms... and poof. But this is just speculation.

Thanks everyone so far for reacting,

G.
 
4 ohms load on +/-40 volt supplies shouldn’t hurt a 100W TO-3 pair, let alone a 150W pair. But if somebody shorted the speaker wires, it could very well go poof.
I don't want to say I don't believe you, but why did the designer bother then to put a really huge transformer in this thing for impedance conversion? as I said, you can switch between 4-8-16 ohms. I must admit, have not much experience with this kind of equipment, but I like to learn more about it.
 
You mentioned the switch initially, but not the transformer. Most of these little powered mixers don’t bother with one - they just put out full power at 4 ohms, and less into higher impedance. Some commercial PA oriented amps have a transformer for the 70V output, which is bypassed for 4-8 ohm use. Some home amps will drop the power supply voltage when switched for 4 ohms. Exotics like McIntosh have output trafos with impedance taps, and operate step-up on all impedances (Primary Z is very low, allowing the transistors to run at low volts and high current). But that’s the exception and very expensive.
 
Here is a picture of the inside:

IMG_20230611_143415373.jpg


The giant structure top left is the power transformer, to the right you see the output transformer. The designer sure did not skimp on ironware. The black wire there is ground, the red is signal out from amp, yellow green and pink go to the impedance selector. It seems there is always a winding in parallel with the speaker output, with taps you can choose with the selector. With setting at 4 ohms (yellow wire) I measure about 0.3 ohms dc resistance between black and red wire. Does this make sense? or should I suspect a fault in this transformer?

Again, any comments welcome.

G.
 
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It sounds about right. The transformer output is always in parallel across the speaker output because it is the speaker output. The impedance selector would select taps on the output side of the transformer.

Are the transformer coupled outputs the only ones available? I'm not well up on this kind of gear but I was wondering if this was like a 70 volt output to be used with speakers that also have a transformer in them.

Like this:
https://www.sweetwater.com/insync/70-volt-systems-made-easy/
 
It only has the two transformer coupled parallel outputs. It is an autotransformer with multiple tappings, a bit like this:

1686516891911.png


So if I understand it correctly, with the switch on 8 ohm with an 8 ohm speaker, the amp tries to deliver 100 watts in this speaker, wich it should be able to. But if I connect a 4 ohm load with the switch on 8 ohm, it will try to deliver 200 watts, by doubling the current, wich would overload the transistors. Or is my logic flawed?
By the way. the amp does seem to have overcurrent/short output protection. The schematic is very much like this:

1686517810150.png


But the VAS has a ccs and not the bootstrapped current source like above.
 
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Yes, it will try to deliver 200 watts, but the power supply droop will keep it to about 150. If there is current limiting it may activate often, sounding terrible. With or without it will put a heavy load on things and may fry the outputs if sustained. Short term it shouldn’t do much to it. But those old aluminum TO-3’s don’t last forever anyway under high power use, even within rating.

Just fix whatever else may be busted, put in 150 or 200 watt-class darlingtons and call it a day.
 
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It's a 'Park Marshall' from the '70's. The darlingtons are MJ3001 / MJ2501. So, the MJ11015 / -16 could be used. Farnell still has the MJ3001 / MJ2501 but not the Motorolas.

https://nl.farnell.com/multicomp-pro/mj2501/darlington-transistor-to-3/dp/9453130

https://www.drtube.com/schematics/marshall/2099u.gif

https://reverb.com/item/3605502-vin...-channel-pa-mixer-desk-w-wooden-side-serviced



Over at the Marshall forum you might perhaps find a schematic.

It's a nice vintage mixer, take good care of it.



Hugo
 
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